The scanning electron microscope is configured to provide a two-dimensional scanned image, which accelerates the electron emitted from an electron source of acceleration type or field emission type to form a thin electron beam (primary electron beam) using an electrostatic lens or an electromagnetic lens, two-dimensionally scans the primary electron beam on the sample to be observed to detect a secondary signal such as secondary electron and reflected electron, that is secondarily generated from the sample irradiated with the primary electron beam, and inputs strength of the detected signal in the display unit such as cathode-ray tube through brightness modulation, which is scanned synchronously with scanning of the primary electron beam.
The secondary signal generated from the sample through the primary electron beam radiation has a wide energy distribution. For example, some of the primary electrons incident on the sample may be elastically scattered by the atom on the solid surface to jump out from the sample surface. Such electron is called a reflected electron which exhibits energy equal to or higher than that of the primary electron beam to a certain degree. Some of the primary electrons incident on the sample may interact with the atom inside the sample. The electron inside the sample is excited and supplied with kinetic energy so as to be released outside, which is called a secondary electron exhibiting energy ranging from approximately 0 eV to 50 eV.
The secondary electron and the reflected electron inherently contain individual unique information owing to different cause of sources. The secondary electron with low energy is allowed to escape from the sample surface only with small depth. This may provide the microstructure of the sample surface with high resolution. The reflected electron with high energy exhibits elevation angular components in various directions. The sample information at each of the respective elevation angles is different. In an explanation described herein, the elevation angle of the reflected electron will be defined as the angle formed between the optical axis of the radiated primary electron beam and an emission direction of the reflected electron from the sample surface. The reflected electron at high elevation angle (high angular component) contains relatively larger volume of surface information in addition to the inside information and the composition information. The secondary electron has a feature of high edge contrast. However, there is an advantage of providing the surface information by observing the reflected electron at the large elevation angle while suppressing the edge contrast. The reflected electron at the small elevation angle (low angular component) provides insufficient information about the sample surface, but is sensitive to the composition information. The secondary electron and the reflected electron emitted from the sample have the aforementioned sample information. Therefore, conventionally, the required sample information is obtained by selecting the secondary electron signal or the reflected electron signal depending on the sample to be observed for acquiring the image with the emphasized information.
The object to be observed by the scanning electron microscope is widely ranged from the semiconductor device to the biological sample. Mostly, the secondary electron image has been used for observation and measurement of the semiconductor device. As the sample shape has been getting complicated, the demand of using the reflected electron signal is increased to obtain the sample information suitable for the object. The number of the reflected electrons emitted from the sample is smaller than that of the secondary electrons by ⅕ or less. Additionally, the high kinetic energy of the reflected electron may interfere with control of the orbit by the electrode. The generally employed scanning electrode microscope is structured that most of the reflected electrons will impinge on the electrode and wall surfaces of the apparatus before reaching the detector, which cannot be detected. For that reason, the reflected electron image provides insufficient S/N for observation and measurement compared with the secondary electron image. Patent Literature 1 discloses the technique for solving the aforementioned problem, which includes a secondary electron conversion electrode that generates the secondary electrons by the impact of the reflected electrons, and a withdrawing electrode that withdraws the secondary electrons generated by the secondary electron conversion electrodes to efficiently detect the reflected electron generated from the sample at the low angle so as to improve the detection rate of the reflected electron higher than the generally employed case. Patent Literature 2 discloses the technique for generating the sample signal optimally suitable for the observation target by improving the detection rate of the reflected electron using the same structure of the apparatus as the one disclosed in Patent Literature 1, and adding three kinds of secondary signals each containing different sample information (secondary electron, low angular reflected electron, high angular reflected electron) at the respective ratios.